For interrogation of biological systems, one is generally interested in a chemical or physical quantity. For a chemical quantity, typical assays determine the presence or concentration of a protein, antibody, or small molecule anylate, the presence or concentration of a particular DNA or RNA, or even more subtle quantities such as the phosphorylation state of an enzyme.
In general, these biomedically-relevant physical quantities are sensed and turned into a measurable optical or electronic signal. The use of electronic interrogation of biological function can be integrated into a silicon complementary metal-oxide-semiconductor (CMOS) chip at potentially low cost. However, the issue of how to interface the CMOS chip to the outside world must be resolved.
Many current implantable biosensors require a wire coming out of the patient, or a battery to be implanted. Also, typical biosensors are large and unsuited for a variety of applications that require minimal invasiveness. With respect to wireless transfer of information, antennas are either external, which add to the size of the system, or too big for applications in interrogation of biological systems. Efforts in reducing the size of antennas beyond a certain point are met by known technical drawbacks, which are discussed in further detail below.
RFID Technology
The field of RFID in general is a complex field, with many applications in industry, medicine, and commerce. Generally, overall size reduction is not the primary goal in industry applications; rather, cost is the most important factor. In addition, reducing the size of antennas often runs against textbook figures of merit, such as antenna gain, efficiency, and impedance.
With respect to RFID chips, companies such as Hitachi have developed technology for progressively smaller die sizes for RFID tags. See Usami, Sato et al., ISSCC (2003); Usami, Tanabe et al., ISSCC (2007). For example, the Hitachi μ-chip is 50×50×5 μm. See Usami, Tanabe et al., ISSCC (2007). This demonstrates the feasibility of small (microscopic) chips for RFID. However, the antenna used with the Hitachi μ-chip was external and added significantly to the system size. Although the Hitachi work has demonstrated very small die sizes for the memory, the antenna must be external, and is typically cm or so in size. This is generally achieved via an off-chip antenna.
Research regarding on-chip antennas has demonstrated the ability to fabricate smaller RF antennas on the same chip as the signal-processing components. Using either GHz near-field antenna or MHz inductively coupled coils, researchers have shown of order 1 mW of available DC power on chip (from the RF field) in a area of order 1 mm2. See Guo, Popov et al., IEEE ELECTRON DEVICE LETT. 27(2), 96-98 (2006) (“Guo reference”). In the Guo reference, the researchers used an OCA operating at 2.45 GHz. The on-chip circuitry used the energy from the incoming RF field to power itself, so that no battery was needed. There, the researchers showed that 1 mW was available to power the on-chip circuitry, and the antenna size was roughly a few mm by a few mm. As can be seen from FIG. 1, the RFID tag chip 10 area is ˜1×0.5 mm2 and the antenna 12 is still much larger than the active circuitry of the Hitachi microchip 14 (which is not part of the RFID tag chip 10, but is only inserted for reference as to scale). Thus, while the Guo reference has demonstrated a major advance in integration and size reduction (compared to the cm scale external antennas typically used), there is still vast room for improvement in miniaturization of this RFID device.
Small Radios
FIG. 2 illustrates an embodiment of a carbon nanotube radio 20. This comprises an AM demodulator 22 made of a single carbon nanotube (a molecular tube with radius of order 1 nm). However, the external antenna 24 is several cm in length, and the audio amplifier, speaker, and power supply (battery) are off the shelf, so the entire system volume is of order 10−3 m3.
Table 1 contains a compilation of some representative sizes for the circuit, antenna, and complete radio system, from various scientific literature. See Bouvier, Thorigne et al., DIGEST OF TECHNICAL PAPERS, 44TH ISSCC, 1997 IEEE INTERNATIONAL (1997); Abrial, Bouvier et al., IEEE J. SOLID-STATE CIRCUITS 36(7), pp. 1101-07 (2001); Hill, Berkeley, Calif., Ph.D.: 166 (2003); Usami, Sato et al., ISSCC (2003); Rutherglen and Burke, NANO LETT. 7(11), 3296-3299 (2007); Usami, Tanabe et al., ISSCC (2007). The values represent estimates only, as most literature does not specify complete system volume. This comparison is meant to give an overview of various technical approaches (and so is not to be considered an “apples to apples” comparison), and to illustrate the state of the art and the relative importance of antenna volume in total system size. From this it is clear that the small circuit size is possible, but having a small antenna size is more challenging.
TABLE 1Estimated circuit, antenna, and system size for variousradios compiled from scientific literatureCircuitantennaSystemsize (m3)size (m3)sizeHitachi1.E−141.E−081.E−08UCI CNT1.E−231.E−051.E−03RadioFrance- 1E−09 1E−09 1E−09TelecomSmart3.125E−09  1.E−061.E−06DustSMSNANA1.E−06BioRasisNANA5.E−09ISSYSNANA1.E−06Potential 1E−14single-chip radioVolume of 1E−18single cellPotential 1E−21nano radio
Table 1 also estimates the size of a possible single-chip radio using “COTS” (commercial off the shelf) technology, as well as possible advances using nanotechnology. In FIG. 3, the system size and single cell size of various existing and possible radio systems are shown.
The field of antenna studies which are smaller than an electrical wavelength is termed electrically small antennas. Researchers have proposed using novel quantum properties of a single carbon nanotube to make a resonant antenna with size about 100 times smaller than a classical dipole antenna for a given frequency. Such a concept is indicated schematically in FIG. 4 (Burke, Yu et al., IEEE TRANS. NANOTECHNOL. 5(4), 314-334 (2006)), where a nanotube antenna 40 is shown.
While the technology to build such prototype antennas exists (Li, Yu et al., NANO LETT. 4(10), 2003-07 (2004); Yu, Li et al., CHEM. MATER. 16(18), 3414-16 (2004)), the predicted losses due to ohmic currents in the arms of the antenna are severe. In principle, this loss can be overcome by higher intensity input radiation. However, this could result in significant heating of the antenna itself and possibly the surrounding tissue.
An approach to the absorption of RF power is to use it as a local heater, which can be used to effect biochemistry at the nanoscale for nanotechnology investigations and potential therapeutic applications. This is another form of “RF remote control” of biological function, which uses heat rather than circuitry to control chemistry. Two examples using various forms of RF nano-heaters include: therapeutic heaters and RF remote control.
In various other applications, however, a heating in the antenna may be undesirable or inappropriate. Also, as discussed above, there are other practical challenges and tradeoffs associated with attempts to decrease the size of an antenna. As an antenna gets smaller, textbook antenna metrics are sacrificed (e.g., antenna gain, optimum impedance, antenna Q schemes, and reader power). The losses in efficiency reduce the range of the antenna, which may be unacceptable for many applications in electronics.